CRUSTACEA. 097 



Fig. 138, being that of a mandible, with its feeler, or palpus; 

 Figures 139, 140, and 141, representing the fust, second, 

 and third pairs of feet-jaws; and Fig. 142, the first pair of 

 true feet. It would thus seem as if the same constituent ele- 

 ment of the fabric is converted by nature into the one or other 

 of these organs, according as best suits the exigencies of each 

 particular case.'^ 



In the lobster, the crab, and many other analogous Crus- 

 tacea, the foremost pair of true feet are also modified to suit 

 a particular purpose; the pincers which terminate them being 

 expanded into a claw, and constituting a powerful oro-an of 

 prehension, and a formidable weapon of offence. It resem- 

 bles a finger and thumb in its power of grasping and strongly 

 compressing any object on which it seizes; and, to enable it 

 to do this with more effect, the inner edges of both parts of 

 the claw are notched or serrated. 



The large portion of shell which is consolidated into one 

 piece, and covers the upper part of the body, is termed the 

 shield, or carapace. The tail of the crab is very short, and 

 is united with the body, appearing as if it had been folded 

 under it. The feet-jaws are particularly large, but short: 

 the articulations of the feet are such as to allow of scarcclv 

 any motion but in a transverse plane. This is the cause of 

 the greater facility the crab finds in walking side-ways, 

 which it can do with great quickness when urged by a sense 

 of danger. The lobster, on the contrary, is better formed 

 for swimming than for walking. The hinder part of its 

 body is divided into segments, which play upon each other 

 by a remarkable kind of m.echanism, the margins of each 

 portion overlapping the succeeding segment, and partly en- 

 closing it. The tail is the principal agent used in swim- 

 ming, and the whole force of the muscles is bestowed upon 

 its movements. As it strikes the water from behind for- 



• The labours ofSavigiiy, Audouin and Latreille appear to have established 

 a complete analogy in the respective component parts, not only of the feet, 

 feet-jaws, jaws and mandibles, but also of the palpi and other appendices at- 

 tached to the head, in all the articulated animals, whether belonging to the 

 closes of arachnlda, crustacca, myriapoda, or winged insects. 



